Intelligence transmission system



March 28, 1950 G. L. FREDENDALL ETA; 2,502,213

mmLLlcENcE TRANSMISSION SYSTEM Filed March 24, 1944 2 s sheets-mt 2mir/warme AAAAAA 'Vvvvv Ari-@gwn G. FREDENDALL ETAL INTELLIGENCETRANSMISSION SYSTEM March 28, 1950 Filed March 24, 1944 3 Sheets-Sheet 5Patented Mar. Z8, 1950 INTELLIGENCE TRANSMISSION SYSTEM Gordon L.Fredendall and Alfred C. Schroeder,

Feasterville, Pa., assignors to Radio Corporation of America, acorporation of Delaware Application March 24, 1944, Serial No. 527,920

(C1. FX8-5.8)

3 Claims.

The present invention relates to intelligence transmission systems andapparatus, and more particularly to television systems in whichintelligence signals, such as sound signals, for example, aretransmitted along with image signals on a single carrier for receptionby single channel carrier current or radio receivers.

According to the invention, a signal representing sound, which is toaccompany visual entertainment such as a television program, istransmitted in a novel manner along with the signal representing thevisual information. In the invention in a preferred form, signals ofvarying frequency representing continuous sound are alternated with theintermittent image signals.

The time intervals between the intermittent image signals are generallyprovided by the retrace of the unilateral scanners at the transmittersand receivers, and these time intervals are occupied by synchronizingsignals for each ield or frame of the transmitted image or view. Thesesynchronizing signals, in the preferred form of the invention, occuringduring this flyback time provide for the transmission of sound signalson a pulse carrier in a novel manner.

A still further object of the present invention is to provide for theintroduction of frequency modulated signals in a novel manner betweentrains or series of video signals at a television transmitter.

A still further object of the present invention is to separate frequencymodulated audio signals in a novel manner from an image signal carrierand to produce sound from these separated sigs nale.

Other and more specic objects of the invention will become apparent froma consideration of the following specication and claims in connectionwith the accompanyingdrawings illustrating one preferred form of theinvention, in

which:

Fig. 1 illustrates conventionally in block diagram form a suitablearrangement for a. transmitter embodying the present invention; i

Fig. 2 illustrates in a similar manner a suitable arrangement forreceiving apparatus embodying the invention;

Fig. 3 shows a slight modification in Fig. 2.

Fig. l affords a more detailed illustration of the principal parts ofthe transmitting apparatus;

Fig. 5 and 5a illustrate the combined video or image and audio or soundsignal; and

Fig. 6 aords a more detailed illustration of portions of the receivingapparatus shown generally and diagrammatically in Fig. 2 of thedrawings.

Referring to Fig. 1 of the drawings, reference numeral lil indicates avideo signal source and its associated equipment for generating image orvideo signals according to well known principles of operation. The videosignal source l0 may include a cathode ray camera or transmitterscanning tube of well known design, such, for example, as an Iconoscopeor an Orthiconf It is also to be understood that any form of mechanicalscanning system may be used for producing image or video signalsprovided that a time interval occurs between trains of electricalsignals representing scanned portions of the subject. For example, thegeneration of television signals may be carried out in the mannerdisclosed in a patent to Bedfard, 2,192,121, granted February 2'7, 1940.A suitable synchronizing arrangement is shown in a patent to Seeley,2,256,530, granted September 23, 1941. Smith Patents 2,132,654 and2,132,655, both granted October 11,'1938, show scanning controlgenerators which may be employed with a camera, transmitter, or scanningtube. These patents are mentioned solely because of the fact that theyillustrate one suitable form of a cathode ray scanning system.

Preferably, in practicing this invention, the time interval referred toabove is provided by a slight time separation between the production ofthe series of electrical signals representing successive lines orelemental strips of the subject. If, for example, it is assumed that thesubject being analyzed is scanned over its entire area at a repetitionrate of 30 complete frames per second and that an odd line doubleinterlacing method of scanning is employed so that each image elcl isformed in 2621/2 lines, the line frequency will be 15,750 lines persecond. The entire time required to scan each elemental strip or line ofthe subject matterwill be ofthe order of l/iam of a second (about 65microseconds), and approximately' n; of this total time is occupied bythe retrace or lyback of the scanning beam. This portion of the timeallotted to the scanning of each elemental 3 strip may, it isunderstood, be varied within wide limits and may be made to conform toany standard which may be set up for commercial or other reasons. Itwill be understood that a complete frame may be scanned at one timewithout resorting to interlaced scanning or that even line interlacedscanning may be employed. Higher delity sound can be obtained byincreasing the number of scanning lines so that the number of intervalsof transmission of sound energy per picture field, assuming the same eldrepetition rate, will be higher, thus with vertical line scanning theupper frequency register of the transmitted sound would be increased inthe illustrative example rom approximately 7' kilocycles toapproximately 10 kilocycles.

The television scanning device included in the video signal source l0 isoperated to test or analyze the varying intensities of light and shadowyon each elemental area of the subject, and if it is assumed thatthedevice l0 is a cathode rai7 tube of the sametype shown by the abovenoted Bedford patent and others,.this tube may'operate under the controlof e. horizontal synchronizing signaland a vertical synchronizing orframing signal each of Which generally is eiectively of saw tooth Waveform. This saw tooth Wave form is adapted to control the delectingfields governing the motion or deflection of the cathode ray Within thescanning device ill. nizing' generator Il is the source. of these. sawtooth deflecting currents or voltages. It will be understood. thatamaster pulse. generator of any known or desired type. is incorporated inthe synchronizing signal generator Il and synchronizes i f' the;deecting: operation. In the illustrative example the horizontalsynchronizing? source will provide a frequency of 15,700 cyclesper'second (assuming a 525 line scanning of each image frame) and thevertical synchronizing source will provide a frequency of 60 cyclesxpe'rsecond. Special provisions for interlacing, if it is employed assuggested, may be provided in manner vknown in the art. Thesynchronizing signal generator Il may serve to provide a blanking biasin a well known manner (not shown) so as to cut 01T transmission fromthe video signal source l0 during the horizontal and vertical retrace orblanking periods.

It will be understood by those skilled Vin the art r thatV the videosignal source I0. combines the horizontal and vertical synchronizingcontrol pulses and the image signal variations and shapes the combinedresultant Wave or composite signal for application kto an amplitude :i

Ipoints orstations i's equally suitable.A

Thespeech or other audible signals, which ac- .'company the pictorialrepresentation transmitted by the television scanner in the video signalsource Ill, are caused to influence the sound signal device shownconventionally'as amicrophone 23.

The synchroimage.

The audio signal from the microphone is supplied to a frequencymodulating oscillator 24. The frequency modulated output from theoscillator 24 is supplied to the mixer I8 under control of a keyer 25and is combined with the composite video signal in the mixer I8. Thedetails of the operation of the keyer, which is controlled by a, keyingpulse generator 28, will be described more in detail hereinafter inconnection with Fig. 4 of the drawings. The keying pulse generator 28controls a vertical synchronizing slotter 29 which makes provision foradding the sound modulated signal during the vertical synchronizingoperation by carrying the signal level in the negative direction duringthe time in which a discrete portion of the sound signal is to be addedto the composite signal by the mixer I8. The signal representing soundis added during the horizontal blanking intervals by its additiondirectly to the video signal without substantially altering the.customary synchronizing pulses.

Fig. 2 of the drawings shows diagrammatically the equipment of atypical'receiver which may be employed for receiving image and soundsignals radiated by the transmitter I2 of Fig. 1. As the received energyincludes sound, synchronizing and video or image signals, it isdesirable that these signals be appropriately separated one from theother. The synchronizing signals are separated from the video signals inany well known manner which may be either by wave form selection, byamplitude selection, or by frequency discrimination. The receiver ofFig. 2 is illustrated as being of the usual superheteroclyne type havinga radio frequency and intermediate frequency amplifier 3i. The-outputfrom the amplier 3l is passed to a second detector 32 which recovers thecomposite signal shown in Figs. 5 and 5c and applies it to a videoamplifier 36 and a synchronizing signal separator 38. Signal separationis accomplished in the synchronizing signal separator 33, and controlsignals are supplied from this separator to the deflection generator 4l.Horizontal and vertical sweep frequencies of appropriate wave' form areapplied to the deiiection system 42 of an image producing device such asa fluorescent screen cathoderay tube 43. The video signals are suppliedto the tube 43 from the output circuit of the video amplier 30 which isalso connected to the` second detector 32.

The vertical synchronizing signal is adapted to control the position atWhich the electro-optical eiects in the tube 43 are produced, and itwith the blanking signal also serves to render the video signal ampliner36 or the tube 43, or both as desired, inoperative during the returnline. Similarly, the horizontal synchronizing system which controls theline scanning the line scanning rate is adapted along with thehorizontal blanking signal to render either the image signal ampliiicr36l or the image tube 43 inoperative during the return line or retraceperiod vbetween the reproduction of successive. elemental strips. of theThisv typeof control we1l...known and does not constitute a part of thepresent invention .except insofar.: as it'. cooperates with the generalcombination ofthe variousothei' elements to include novel subjectmatter..

The'receiver equipment thus far' described may be of any type chosen toreceive video signals from the transmitter of Fig. 1. The soundreceiving and reproducing arrangements in accordance With the invention,now to be brieily described, aref added without disturbing the usual 5gfunction of the electro-optical. `image' producing equipment.

Une arrangementlvin accordance Wtlnthe lnfvention for effecting therecoveryof. the frequency Imodulated sound signal andA its. conversion.to audible sound effectszis shown diagrammatical-ly in Fig. 2. Theintermediate frequencyoutputof 'thelamplifiertfl*isI fedby way of aconnection 48 to avselector ill. Video and: synchronizing signalsvlaswell as the*r frequency modulated sound lsignals 'appear asmodulations-of the intermediate fre:- `quency in theconnecti'on 48 and.therefore the function of" theselector isto. select-the intervals offrequency modulated sound. from. the: intermediate frequency signal.`Operationpof thesejlector 5| is controlled by arfrequencygenerator,such. as a multivibrator 53., which issshown; as being controlled,inturn, from the deectionzgenerator 4| by wayl of a. connection 5E;Ablanking signal is applied to.- the image producmg tube 43 by way of aconnection 58'1fr0ma blankingsignal generator 59. The blanking signalgenerator may 'conveniently be.v` controlled from` the; multivibrator53.

The selector 5|1 passes Vonly-thezportions Yoi/the video intermediatefrequency signal. containing the frequency modulated sound signalintervals yto -a limiter iii! anduaidiscriminator E21. The output fromthe dscriminator 62, 4which. is inform of amplitude modulated pulsesrepresenting:

"half of the horizontal' synchronizing frequency 'and serves to pass:only the:frequencyfcomponents lying `withintherange of the sound.frequencies impressed on. the microphone 23` (Figi.v 1'). Thecharacteristics ofthenl-.ters 6.3 and 12 (.Fig. 4) are preferablysimilar.

Another arrangement for effecting recovery of Ithe frequency modulatedsound signal and` its conversion toaudible sound eifects is: showndiagrammatically inA Fig. 3 of the. drawingsl in which 4portions of theapparatus similar to Fig. 2 are omitted forv the sake. of convenience.The video signalfrom. the video signal amplifier 35' is fed to. theselector 51a by Way4 of a connection 65. The selector 5| a, which issimilar to the selector 5| to bedescribed.` more in detail hereinafter,

Apasses the. video signal toalimiter 6:5 and adisil.crim `u'iator 6l-The: limiter-and discriminator of Fig. 'operateat thel frequency of thecenter frequency produced by the oscillator 24:0f. Fig. l of thedrawings. The output of. the discriminator '6.1' is applied to the soundreproducer 64 by way of aloWfpa'ss filter 68.-.

Amorey detailed descriptionof the soundreproducing, arrangements and'their operation, shown diagrammaticallyby Figs. 2 and 3v of. theydrawings Willbe given following the detailed del- :scriptiorrl of the:sound pickupfsystenr of. Fig; l and it operation and particularly bythe-:description of Fig. 6 and its operation;y

Referring; novvtog Fig. ly .ofi the-drawings: vll-rich.l

,.withrtlie example given-above; beffI-8'ldcycles1per i* second.` Theaudio. input applied tothe connec- 'tionSS modulates an oscillatorcomprising circuits associated with avacuum tube 16.

This tube '|6and other tubes employed. in the :apparatusy showny in thedrawings are,l it is understood,y provided with the usual means 79; suchas thel connecting pins orthe like on the vacuum tubeVbase-'whichcooperate with a` tube socket, for connecting the terminals of thecircuitelemen-ts to the elements -contained in the vacuum tube;

The oscillator tube 16 in combination with a tube 82', which operates asa reactance tube, provides. the output which is frequency modulated bythe-sound. signal appearing at the sound input terminali 69. The mannerin which the reactance tube 82 functions to vary frequency of theoscillator 'I6 will appear from the-following description oftheoscillator and its function.

The electrode 86 of the tube 'I6 serves as the plate of the portion ofthe tube which functions as zin-oscillator; and the grid 88 serves asthe oscillator grid. Closely coupled coils. Si anclSZ., which may inaccordancewith usual practice be combined. together in av tapped coil,in conjunction with a grid leak and condenser combination 93, causeoscillations to be generated cfa frequency determined by a condenser 96which is effectively inzseriesA with the capacity furnished by the re--actance tube 82. A capacitance S7 serves asa bypass between theelectrode 86- and the cathode .98 at the frequency' of the generatedoscillations. Andamped. tuned circuit composed of an inductance lill anda resistance |2 serves' as a plate loadand increases thevoltage outputVfromtiie tube'l'l'.`

The grid ||l||1 of the reactance tube 82 is ccnnected to the audiosignal source, which, in. the example given, comprises the lter 12,through an inductance H31 which serves as a choke. Oscillations from theoscillator portion of the tube 'i6` are'fed tothe grid lull through acapacitance and resistor combination A leakresistor ||3 in combi-nationwith the input capacitance |09 of the tube `82 determines the phase ofthe voltage fed back to the grid |04. The plate loa-d for the tube 82 isin the form of a choke I4 which serves to keep theV high frequencyoscillations out of the plate voltage source. Thev audio source variesthe frequency of the oscillator 16 by varying the oscillator tuningcapacity which is provided in part by the tube 82. The magnitude of theeffective inductive reactance of the tube varies in accordance with thefrequency of the amplitude variations from the audio modulating source.

The frequency modulated output of the frequency modulated oscillator 16,appearing in a connection liliis applied by Way of a coupling condenserl to the grid |8fof a relatively xed frequencyy oscillator lll. Thefrequency generated` by the oscillator |25A differs fromv4 the frequencygenerated by the. oscillator i6. by the value: of the desired frequencymodulatedA carrier.v Eor. example; the centerl frequency of theoscillator '|81 mayl be. set at 50. me'gacyclesand the:.frequea'icylofthe oscillatorV l 2| may b e. set. at 5.21 megacycles so thatv thecenter frequency of the frequency modulated carrierv for the derivedsound signals Willbe- 2 megacycles. The circuits which arev associated.with ther` electrodes of the oscillatorvtube- |21. are or may be similarto those employed. in connectionwith the tube 'i6 and comprise a. gridleaky and condenser |23 anda .tapped inductance |24. Only-theinstantaneous difference frequency of oscillators i l2| and/legis `usedsubsequently. Thus, inductance 2|@ together with the total distributedcapacitance of tubes and circuits effective across 2|0 are resonated atthe said difference frequency. This difference frequency is amplified inan amplifier tube |26 of any desired type connected in any -in itsentirety.

The keyer tube 53| operates to pass the frequency modulated sound signalto the transmitter output at appropriate times under the control ofbiasing pulses applied to another of its grids |33. The manner ofobtaining and timing 4these pulses will now be described in detail.

The keying pulse generator 28 (Fig. l) may,

.for example, comprise a multivibrator |38 (Fig. .4) and a delayed pulsegenerator shown a vvacuum tube iti and its associated circuits. The

multivibrator |38 is of the usual type comprising two tubes H53 and |44.The output of this multivibrator is locked in with the horizontal .il

driving impulses and they are provided by way of a connection |136(Figs. 1 and 4). Impulses employed to interrupt continuity of thevertical synchronizing pulse, as will be explained in con-1 nection withFigs. 5 and 5a, are derived from the vertical synchronizing pulseslotter indicated -by reference character 29 on Fig. 1 and shown more indetail by Fig. 4 as comprising a multivibrator |1253 and a delayed pulsegenerator comprising a vacuum tube l5! and its associated ..1

circuits. The tube |51 is operated in substantially the same manner asthe tube Mii. The

'output connection |53 from the tube |5| is shown as being connected tothe synchronizing generator The output from the multivibrator E38 isapplied to the grid |56 of the tube Uli by way of 'a capacitance |58. Arelatively high positive 'bias is applied to the grid |56 by way of agrid resistor |59. Theduration of the pulse applied to the grid |33 ofthe lseyer tube |35 is determined by the capacitance and resistance ofthe elements 58 and |59. The positive bias on the grid of the tube lilicauses it to draw grid current until it biases itself oi. The voltagedrop in the grid resistor takes up the positive bias applied from thebattery. The pulse output of the tube 16| is initiated from the pulseoutput of the multivibrator |38. This arrangement for obtaining timedelay and width control of a pulse -is disclosed in a patent to Karl R.Wendt, No. 2,313,906, issued March l5, 1943.

The pulse from the tube |4| is, as previously stated, applied to thegrid |33 of the tube |3| as a positive bias and causes this tube tobecome conductive at a time after the horizontal-synchronizing pulse hasceased. The frequency modulated sound carrier from the amplie'r v|25 istherefore transmitted to the amplifier tube |61,

which serves as a mixer for the video signal applied over the connectionI6 (Figs. 1 and 4) by way of an amplifier |63 if additional amplifica'-tion is desirable or necessary. Intervals of frequency modulated soundare added to the video signal 'at the point |66, and the output `of theThe reference character 2t is :amplifier |6| vcontainingbothvideo andsound signals is applied to the amplitudemodulated transmitter l2 forradiation. A cathode resistor it and a condenser |65 serves to bias thetube |3| off during transmission of image signals to the point |66 sothat there is no current in its plate circuit.

The wave form of the signal appearing at the control grid |68 of theamplifier tube |65 is shown in Figs. 5 and 5a of the drawings. Thehorizontal synchronizing pulses, indicated by the reference character|69, occur at regular intervals eiectively without interruption, and thekeyer 26 adds the frequency modulated sound signal indicated byreference character |'|I after the occurrence of each horizontalsynchronizing pulse and for the interval of time determined by theoperation of the tube I4! as explained previously. The addition of thefrequency modulated sound pulses shapes the Wave form of the signal asshown in Figs. 5 and 5a without any necessity for additional apparatusfor slotting the wave form at the time of transmission of the soundsignal or changing the blanking level. The adjustment of the tube Ml issuch that the termination of the blanking period is sharply defined inthe usual manner by the end |12 of the blanking interval. The imagesignal variations are indicated at H3 and contain the intelligencenecessary to recreate the image at a receiver. A vertical synchronizingblanking interval is shown by way of example, and it includes horizontalsynchronizing pulses |69 and equalizing pulses |74 in the usual manner.The vertical synchronizing pulse i'ii is slotted by the action of thevertical synchronizing signal slotter |48 and the time delayed pulsegenerating tube |5|. The slots which are produced in this manner areindicated by reference character |18 and extend only to the blankinglevel asl indicated by the crosshatching of these slots on the drawing.llhese slots are produced in proper time relationship with respect tothe complete signal by means of the delay after the normal synchronizingperiod produced by the tube |5|. At the time of production of theseslots, the tube lill causes the keyer 26 to be active and to add thefrequency modulated sound signals il a to the video signal.

It will be seen that the transmission of sound is essentially the resultof transmitting a frequency modulated wave at intervals determined bythe frequency of horizontal driving. The timing of these intervals oftransmitted sound is, because of the manner of transmission employed,not especially critical since the beginning of the interval may occur ata varying time after the synchronizing pulse |68 ceases and is to endslightly ahead of the end |'i2 of the blanking interval. This timerelationship with respect to the horizontal synchronizing pulses andequalizing pulses in the vertical blanking region is also maintained.

As stated above and as is apparent from au inspection of either Fig. 2or Fig. 3, the portion vof the television receiver employed to receivesignals transmitted from the antenna 2| of Fig. 1 and to recreate theimage scanned by the television scanner is or may be of any well knowntype without departing from the spirit of the invention. The soundsignal apparatus added to the image receiver in accordance with Fig. 2of the drawings will now be described more in detail with reference toFie. 6. l

A vacuum tube |86, forming theprincipal part of the selector device(Fig. 2), serves to connect the sound signal channel with the soundreproducer at the proper time intervals so'that the video signal willnot cause interference in the sound channel. The video signal containingthe synchronizing pulses and the frequency modulated sound intervals |1|and |1|a is applied to the control grid |88 of the tube |86 by way ofthe connection 48 which may receive energy from the plate circuit of theintermediate frequency amplifier 3| or the plate circuit of the videoamplifier 46 as shown in Fig. 3 of the drawings. The tube |86 is biasedoff by a biasing potential applied to its grid |9| from themultivibrator 53 by way of a coupling condenser |92 and a connection|93.

The multivibrator 53, which may be of any desired type, is synchronizedby a signal obtained from the horizontal deflection circuit for theimage producing tube 43 by way of a connection 56 as mentioned above.While this controlling pulse or wave source is indicated forconvenience, pulses of any kind from the deflection generator 4| may beemployed. The output from the multivibrator 53 occurs in timedrelationship with the sound signal intervals being received so that thetube |86 is conducting only during these intervals. This result isaccomplished by appropriately choosing the circuit constants of elements|96, |91, |98, and |99 as well as by suitably adjusting controls and 202of the multivibrator. The control 20| of the multivibrator is a variablegrid leak which determines the lspeed at which the multivibrator runs orits frequency, and hence this is the control used for synchronizing themultivibrator. The Width of the selecting interval is controlled by theadjustable grid lead 202 and determines the time that the tube |86 isconductive. The capacitance |96 together with the resistance |91 incombination with the resistance |98 and the capacitance |99, togetherwith the setting of the adjustable grid leak 20|, determines the timingof the multivibrator 53. The capacitance |96 and the resistor |91 serveas a differentiating circuit to produce control pulses which are in stepwith the horizontal deflection pulses, but these pulses obtained bydifferentiation occur slightly later than the horizontal synchronizingpulses'. One method of obtaining the time occurrence of thesedifferentiation pulses has been indicated in the foreground, mainly toemploy the saw tooth horizontal deflection wave applied by way of theconnection 56 inasmuch as each sloping portion of this Wave startsfollowing the occurrence of a horizontal synchronizing pulse.

The resistor |08 serves the additional purpose of keeping multivibratoroscillations out of the receiver circuits.

A resistor 204 in the connection |93 provides for Wave shaping of thegrid biasing pulse from the multivibrator when the tube |86 draws gridcurrent at its grid |9|.

The selector 5| is coupled to the limiter 6| which in turn is connectedto the discriminator 62 as stated above. The limiter and discriminatorare of any well known type and perform the function usually performed insuch equipment as a frequency modulated sound receiver. The limiter anddiscriminator are tuned to the rest or center frequency of the frequencymodulated pulses i. e. the difference frequency of oscillators 16 and|2|, Fig. 4, or to the difference between the corresponding intermediatefrequency genin Fig. 6 as being amplified by an amplifier 206- beforebeing applied to the filter 63.

The blanking pulse derived from the multivibrator 53 is amplified in atube 208 and is applied to the image producing tube 43 by Way of theconnection 58 to insure blanking of the image reproducing tube duringthe sound transmission time. The tube 208 may, it is understood, besubstituted for the blanking signal generator represented by referencecharacter 59 in Fig. 2 of the drawings. The blanking signal from thetube 208 is preferably applied to the cathode of the image reproducingtube and may be in addition to or may be substituted for the usualblanking connections.

Various alterations and modifications may be made in the presentinvention without departing from the spirit and scope thereof, and it isdesired that any and all such modifications be considered within thepurview of the present invention as defined by the .hereinafter appendedclaims.

Having now described the invention, what is claimed and desired to besecured by Letters Patent is the following:

l. Apparatus for receiving Video and sound signals multiplexed on asingle carrier comprising an image producing device, a local pulsesource, a relay device, means to supply a biasing potential to saidrelay device to produce a cut olf condition in said device under controlof said local pulse source, means periodically to render said imageproducing device inoperative under control of said local pulse source,means to supply a frequency modulated wave to said relay device fortransmission thereby during time intervals when said device isoperative, means for limiting the amplitude of said received wave, meansfor converting frequency variations of said wave into amplitudevariations, and means for translating said amplitude variations intoaudible effects.

2. Apparatus for receiving video and sound signals multiplexed on asingle carrier comprising an image producing device, a vacuum tube relayassociated with said apparatus, a local pulse source, means to supply arelay cut oli" biasing potential, means to connect said bias supplymeans to the vacuum tube relay, means periodically to render said imageproducing device inoperative under control of said local pulse source,means to connect a source of signal modulated wave trains to the relaydevice for transmission thereby during the time intervals when saidrelay device is operative, an output circuit, means for connecting saidoutput circuit to the vacuum tube relay, means for converting frequencyvariations of said wave into amplitude variations and means fortranslating said amplitude variations into audible effects.

3. In a signalling system the method of transmitting simultaneouslyoccurring signals of different kinds upon a single carrier frequencywhich comprises producing intelligence signal trains variable inamplitude between lower and upper limits, producing additionalintelligence signals of a different kind, producing oscillations andvarying the frequency of said oscillations in accordance with variationsin said different kind of intelligence signals, producing otheroscillations and obtaining a beat frequency varying in frequency inaccordance with the frequency 11 I2 vaifatonsofsaidkrstnamedoscillations, .-pro- Y Y f ducng a, carrier vx/ave, andmodulating `said UNITED STATES PATENTS carrier Wave-by annte11igence`signal trainrand Number .Name ADate affrequ'ency varying signal, andsuccessively and 2',075,0'71 -Usselman -Man 30, 1937 cyclcally repeatingthemodulatng'process. 5 2,039,539 vBedford. Aug. 10, 1937 GORDON vL.FREDENDALL 2,227,108 'Roosenstein .'Dec. 31,1940

ALFRED C. lSCHIRDEDER. 23254g435 `L0ughren vSept. 2, 1941v Y Yy'2,296,919 .Goldstne Sept. 29, 1942 REFERENCES CITED '421,326,515Bartelink 'Aug. 10, 1943- le Gef ences are of record in the 25391376Fredendan Dec. 25, 1945

